Optical head including a semiconductor laser having a non-scatter incident area

ABSTRACT

An optical head for optical recording and reproducing apparatuses is disclosed. The optical head comprises a semiconductor laser, a mirror for reflecting a light beam emitted from the semiconductor laser, a diffraction grating for dividing the light beam into zero order, plus and minus first-order beams. It also comprises a holographic diffraction grating being arranged at an optical path of the beams divided by the diffraction grating, an objective lens for focusing the beams on a disk and causing the beams reflected from the disk to be incident on the holographic diffraction grating to diffract the beams, and a photodetector for detecting a tracking error signal by receiving the beams diffracted by the holographic diffraction grating. The semiconductor laser has a beam incident area onto which the beam reflected from the disk is incident. The beam incident area acts to scatter or absorb the incident beam. As a result, the optical head is able to obtain a stable tracking error signal.

FIELD OF THE INVENTION

The present invention relates to an optical head for optical recordingand reproducing apparatuses.

BACKGROUND OF THE INVENTION

An optical head integrating a light source and a photodetector has beenproposed. The optical head disclosed in Japanese Patent Laid-Open35737/1989 is an example. FIG. 13 shows a perspective view of theoptical head disclosed above and FIG. 14 shows a schematic view of itsoptical system. The optical head chip 1a has first and secondphotodetectors 9 and 10 having five regions respectively on first andsecond substrates 2 and 8. The second substrate 8 is mounted on thefirst substrate 2. A semiconductor laser chip 5 is mounted on the secondsubstrate 8 and a monitoring photodetector 4 is also formed on thesecond substrate 8.

A prism 11 is set on the first substrate 2. The prism 11 has atrapezoidal section, its grade plane 11a acts as a mirror and itsvertical plane 11b faces to a exit of the semiconductor laser chip 5. Afirst diffraction grating 12 is formed on a top plane 11c of the prism11. A second diffraction grating 13 is arranged above the firstdiffraction grating 12.

The outline of operation of this optical head is as follows: A beamemitted from the semiconductor laser chip 5 is reflected by the gradeplane 11a of the prism 11 and incident onto the first diffractiongrating 12. The diffraction grating 12 divides the beam into threebeams, a zero-order beam for reading information and plus and minusfirst-order beams for detecting a tracking error. The three beams passthe second diffraction grating 13 and are focused by an objective lens18 on a disk 19. The three beams focused on the disk 19 are reflectedfrom the disk 19 and are incident onto the second diffraction grating 13through the objective lens 18. The incident beams are divided into plusand minus first-order beams by the second diffraction grating 13 andtransmitted to the first and second photodetectors 9 and 10.

Because the first and second photodetectors 9 and 10 are arranged infront of and behind the exact point of focus as shown in FIG. 15, spotsizes imaged on the photodetectors 9 and 10 associated with the shift ofthe focus point on the disk 19 cause the reverse change to each other.The focus error is detected by the beam size method utilizing thisphenomenon. The tracking error is detected by a tri-beam methodemploying the plus and minus first-order beams for the trackinggenerated by the first diffraction grating 12.

When the semiconductor laser chip 5 is arranged in close proximity tothe prism 11 to miniaturize further the integrated optical headmentioned above, it follows that either of the two first-order beams fordetecting the tracking error reflected from the disk 19 passes thesecond diffraction grating 13 and is reflected from a surface faced tothe second diffraction grating 13 on the semiconductor laser chip 5 andreturns again onto the disk 19. The fact that the beam returnedinterferes with the two first-order beams for detecting the trackingerror signal and the tracking error signal causes an offset as has beendisclosed in Japanese Patent Laid-Open 24031/1986. For avoiding thisinterference, a method forming an optical shielding which scatters orabsorbs the light at the exit of the semiconductor laser has beenproposed in Japanese Patent Laid-Open 24031/1986.

However, in the miniature optical head integrating the light source andthe photodetector as mentioned above, because either of the twofirst-order beams for detecting the tracking error reflected from thedisk 19 is incident onto the surface faced to the second diffractiongrating 13 on the semiconductor laser chip 5 or onto the surface locatedon the opposite side from said semiconductor laser chip 5 with respectto the grade plane 11a acted as the mirror and faced to the diffractiongrating 13, the method proposed in Japanese Patent Laid-Open 24031/1986can not prevent the interference.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical head foroptical recording and reproducing apparatuses which has a light sourceand a photodetector integrated and is able to produce a stable trackingerror signal.

A preferred embodiment according to the present invention comprises:

a semiconductor laser;

a mirror for reflecting a light beam emitted from said semiconductorlaser so as to deflect an optical path of said light beam;

a diffraction grating which is arranged in close vicinity to saidsemiconductor laser for dividing said light beam reflected from saidmirror into three beams in which there are zero-order, plus and minusfirst-order beams;

a holographic diffraction grating being arranged at an optical path ofsaid three beams divided by said diffraction grating;

an objective lens for focusing said three beams transmitted through saidholographic diffraction grating on an optical recording medium andcausing said three beams reflected from said optical recording medium tobe incident on said holographic diffraction grating so as to diffractsaid three beams; and

a photodetector for detecting a tracking error signal by receiving alight beam diffracted by said holographic diffraction grating;

said semiconductor laser has a beam incident area, formed on a surfacefaced to said diffraction grating, to which either of said twofirst-order beams reflected from said optical recording medium passingthrough said objective lens, said holographic diffraction grating andsaid diffraction grating is incident without passing said mirror.

The present invention itself, together with further objects andattendent advantages, will best be understood by reference to thefollowing detailed description of the preferred embodiments taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of an embodiment of an optical head according tothe present invention.

FIG. 2 is a sectional view of a semiconductor laser chip mounted onsubstrate illustrated in FIG. 1.

FIG. 3 is a schematic view of an optical system of the embodiment spoonin FIG. 1.

FIG. 4A and 4B are schematic views of a diffraction grating and aholographic diffraction grating of the embodiment according to thepresent invention.

FIG. 5 is a front view of another embodiment of an optical headaccording to the present invention.

FIG. 6 is a sectional view of a semiconductor laser chip mounted on asubstrate illustrated in FIG. 5.

FIG. 7 to FIG. 12 are schematic views of the semiconductor laser chipand its environs according to the present invention.

FIG. 13 is a perspective view of the conventional optical head.

FIG. 14 is a schematic view of the optical system of the conventionaloptical head.

FIG. 15 is a sectional view of the conventional optical head.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a front view of an embodiment of an optical head accordingto the present invention and FIG. 2 shows a sectional view of asemiconductor laser chip mounted on a substrate illustrated in FIG. 2.

On a substrate 31, photodetectors consisting of five region 33 and 34and a mirror 35 made of the chemical anisotropic etching are formed. Asemiconductor laser chip 32 is mounted at the position faced to themirror 35.

A feature of this embodiment is the fact that a wire bonding portion 41joining a wire 39 to an electrode surface 40 is formed on the electrodesurface 40 of the semiconductor laser chip 32.

Referring to FIG. 1, FIG. 2, FIG. 3 showing a schematic view of anoptical system of the embodiment, and FIG. 4 showing schematic views ofa diffraction grating and a holographic diffraction grating of theembodiment, the operation of the optical head will be explained in thefollowing paragraphs.

A beam emitted from the semiconductor laser chip 32 is reflected by themirror 35 as indicated by the arrow and incident onto a diffractiongrating 36a formed on the bottom of a transparent optical element 36.The diffraction grating 36a divides the incident beam into three beams,a zero-order main beam for reading information and plus and minusfirst-order sub beams for detecting a tracking error. The beams passholographic diffraction grating 36b formed on the top of the transparentoptical element 36 and are focused on a disk 38 by an objective lens 37.The focused beam is reflected from the disk 38 and incident onto theholographic diffraction grating 36b through the objective lens 37. Theholographic diffraction grating 36b divides the incident beam into plusand minus first-order beams. The divided beams are imaged onto thephotodetectors 33 and 34.

The tracking error signal is produced by a tri-beam tracking method inwhich a difference of intensity between the beams reflected from thedisk 38 is detected by the photodetectors 33a, 33c, 34a, and 34c. Thatis, the tracking error signal TE is given by:

    TE=(G+I)-(H+J)

where G, H, I, and J are outputs of the photodetector 33a, 33c, 34a, and34c.

In FIG. 1 to FIG. 3, the other sub beam 42a reflected from the disk 38is incident onto the electrode surface 40 faced to the diffractiongrating 36a. The electrode surface 40 is P or N type electrode of thesemiconductor laser chip 32. In the beam incident area of the electrodesurface 40, the wire 39 is bonded so as to supply an electric currentinto the semiconductor laser chip 32. If it is only to supply theelectric current, the bonding position can be random and it is normallybonded on the electrode surface 40 near the center. However, because inthis embodiment the wire bonding portion 41 is located on the beamincident area of the beam 42a, the incident beam 42a is reflected by adome of the wire bonding portion 41 and thus it becomes a scatteredlight 43.

Because the scattered light 43 has a very large spread angle compared tothe incident beam 42a, the amount of the scattered light which returnsagain to the disk 38 through the objective lens 37 is lessenedsignificantly. Therefore, it is possible to minimize an interferenceabout the beam for detecting the tracking error signal due to the factthat the light reflected from the electrode surface 40 returns to thedisk 38.

In above optical head, the detection of the focus error is as follows;

The holographic diffraction grating 36b is formed in such a way that thetwo first-order diffraction beams are imaged onto the front and back ofthe photodetectors 33 and 34. Because the spot sizes on thephotodetectors cause the reverse change to each other when the beam onthe disk 38 is defocused, the focusing error can be detected. The focuserror signal FE is given by:

    FE=(A+C+E)-(B+D+F)

where A, B, C, D, E, and F are outputs of the photodetectors 33d, 33e,33f, 34d, 34e, and 34f.

In the embodiment mentioned above there is scattered the sub beam whichis a cause of the interference by the wire bonding portion 41. Thedifferent methods which give the similar effect are as follows:

A method preventing the reflection of the incident beam by giving anonreflection coating 101 on the beam incident area of the semiconductorlaser chip 32 is shown in FIG. 7.

The other method is to scatter or to absorb the incident beam by forminga dome-like projection 102 on the beam incident area of thesemiconductor laser chip 32 as shown in FIG. 8. The dome-like projection102 can form by bonding a resin or the wire.

The other method is to scatter the incident beam or to reflect theincident beam in an unaffected direction or to absorb the incident beamby mounting a prism-like projection 103 on the beam incident area of thesemiconductor laser chip 32 as shown in FIG. 9. The prism-likeprojection 103 can be formed by bonding a microscopic prism or byetching the semiconductor laser chip 32.

The other method is to diffract and to scatter the incident beam byforming a grating-like unevenness 104 on the beam incident area of thesemiconductor laser chip 32 as shown in FIG. 10: The grating-likeunevenness 104 can form by mounting a plate processed in thegrating-like unevenness form on the semiconductor laser chip 32 or byetching the semiconductor laser chip 32 in the grating-like form.

The other method is to deflect the incident beam in the unaffecteddirection by inclining the semiconductor laser chip 32 as shown in FIG.11. It can form by soldering the inclined semiconductor laser chip 32 orby mounting the semiconductor laser chip 32 via a triangular spacer. Twoinclined surfaces 105 may be formed in this method.

The other method is to scatter the incident beam by forming a frostedglass-like roughened surface 107 on the beam incident area of thesemiconductor laser chip 32 as shown in FIG. 12. It can form by mountinga plate having a roughened surface or by giving the toughened coating onthe semiconductor laser chip 32 or by etching the semiconductor laserchip 32 (for example, the roughened surface can be formed by etching agold thin film with the use of an iodine fluid).

FIG. 5 shows a front view of another embodiment of an optical headaccording to the present invention and FIG. 6 shows a sectional view ofa semiconductor laser chip and a substrate illustrated in FIG. 5.

A feature of this embodiment is tile fact that a nonreflection coating51 is given on the substrate 31 as shown in FIG. 5 and FIG. 6. Thenonreflection coating 51 is given on an area of opposite side from thesemiconductor laser chip 32 with respect to the mirror 35. The mirror 35and the photodetectors 33 and 34 are the same as the embodiment shown inFIG. 1.

The operation producing the tracking error signal TE of this embodimentis the same as the embodiment shown in FIG. 1.

In this embodiment, the other sub beam 42b (the opposite sub beam fromthe sub beam 42a of FIG. 2) reflected from the disk 38 is incident ontoa surface 50 which is given the nonreflection coating 51. Because thereis the nonreflection coating 51 on the beam incident area, almost all ofthe incident beam 42b is absorbed into the substrate 31. Therefore, theamount of the light beam which returns again to the disk 38 through theobjective lens 37 becomes approximately zero. That is, it is possible tominimize the interference about the beam for detecting the trackingerror signal due to the fact that the light beam reflected from thesurface 50 returns to the disk 38.

The operation producing the focus error signal of this embodiment is thesame as in the embodiment shown in FIG. 1.

In this embodiment there is absorbed the sub beam which is a cause ofthe interference by the nonreflection coating 51. The different methodswhich give similar effects are as follows:

A method is to scatter or to absorb the incident beam by forming adome-like projection 102 on the beam incident area of the substrate 31as shown in FIG. 8. The dome-like projection 102 can form by bonding aresin or the wire.

The other method is to scatter the incident beam or to reflect theincident beam in an unaffected direction or to absorb the incident beamby mounting a prism-like projection 103 on the beam incident area of thesubstrate 31 as shown in FIG. 9. The prism-like projection 103 can formby bonding a microscopic prism or by etching the substrate 31.

The other method is to diffract and to scatter the incident beam byforming a grating-like unevenness 104 on the beam incident area of thesubstrate 31 as shown in FIG. 10. The grating-like unevenness 104 canform by mounting a plate processed in the grating-like unevenness formon the substrate 31 or by etching the substrate 31 in the grating-likeform.

The other method is to deflect the incident beam in the unaffecteddirection by inclining the beam incident area on the substrate 31 asshown in FIG. 11. It can form by etching obliquely the substrate 31.

The other method is to scatter the incident beam by forming a frostedglass-like roughened surface 107 on the beam incident area of thesubstrate 31 as shown in FIG. 12. It can form by mounting a plate havinga roughened surface or by giving the roughened coating on the substrate31 or by etching the substrate 31 (for example, the roughened surfacecan be formed by etching a gold thin film with the use of an iodinefluid).

These embodiments are used with the mirror formed by etching thesubstrate, but the mirror formed by other materials (glass, metal,ceramics, etc.) can produce a the similar effect.

As mentioned above, the optical head miniaturized by integrating thelight source and the photodetector according to the present invention isable to prevent the interference which was at state on the conventionaloptical head by forming the beam incident area in such a way as toscatter or to absorb the following two incident beams.

1) The sub beam, which is produced by the diffraction grating, beingreflected from the disk, passing through the objective lens, theholographic diffraction grating, and the diffraction grating, incidenton the surface of the semiconductor laser chip faced to the diffractiongrating.

2) The other sub beam, which is produced by the diffraction grating,being reflected from the disk, passing through the objective lens, theholographic diffraction grating, and the diffraction grating, incidenton the surface of opposite side from the semiconductor laser chip withrespect to the mirror on the substrate faced to the diffraction grating.

As a result, it becomes possible to obtain the stable tracking errorsignal, because there is no generation of the. offset in tile trackingerror signal due to the tangential skew of the disk.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects asillustractive and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. An optical head comprising:a semiconductor laser;a substrate having a hollow thereon, mounting the semiconductor laser inthe hollow; a mirror for reflecting a light beam emitted from saidsemiconductor laser so as to reflect an optical path of said light beam,said mirror formed on a side wall of the hollow; a diffraction gratingwhich is arranged in close vicinity to said semiconductor laser fordividing said light beam reflected from said mirror into three beams inwhich there are zero-order, plus first-order, and minus first-orderbeams; a holographic diffraction grating being arranged at an opticalpath of said three beams divided by said diffraction grating; anobjective lens for focusing said three beams on an optical recordingmedium; a photodetector for detecting a tracking error signal byreceiving said three beams diffracted by said holographic diffractiongrating; wherein,said semiconductor laser is mounted on the substratefaced to said diffraction grating, at a place where either of said plusfirst-order beam or said minus first-order beam reflected from saidoptical recording medium illuminates a surface of the semiconductorlaser, said surface of the semiconductor laser illuminated by either ofsaid first order beams is made to absorb or diffuse light.
 2. Theoptical head according to claim 1,a wire for supplying an electriccurrent into said semiconductor laser is bonded on the beam incidentarea.
 3. The optical head according to claim 1,a nonreflection coatingis coated on the beam incident area.
 4. The optical head according toclaim 1,a dome-like projection is formed on the bean incident area. 5.The optical head according to claim 1,a prism-like projection is formedon the beam incident area.
 6. The optical head according to claim 1,agrating-like unevenness is formed on the beam incident area.
 7. Theoptical head according to claim 1, whereinsaid beam incident area isinclining to said illuminating beam.
 8. The optical head according toclaim 1, whereina roughed surface is formed on the beam incident area.9. An optical head comprising:a semiconductor laser; a substrate havinga hollow thereon, mounting the semiconductor laser in the hollow; amirror for reflecting a light beam emitted from said semiconductor laserso as to reflect an optical path of said light beam, said mirror formedon a side wall of the hollow; a diffraction grating which is arranged inclose vicinity to said semiconductor laser for dividing said light beamreflected from said mirror into three beams in which there arezero-order, plus first-order, and minus first-order beams; a holographicdiffraction grating being arranged at an optical path of said threebeams divided by said diffraction grating; an objective lens forfocusing said three beams on an optical recording medium; aphotodetector for detecting a tracking error signal by receiving saidthree beams diffracted by said holographic diffraction grating,wherein,said substrate has a beam incident area formed on a surface,located on the opposite side from said semiconductor laser with respectto said mirror and faced to the diffraction grating, either said plusfirst-order or said minus first-order beam reflected from said opticalrecording medium illuminates thereon; said surface of the beam incidentarea on the substrate is made to absorb or diffuse light.
 10. Theoptical head according to claim 9,a wire is bonded on the beam incidentarea.
 11. The optical head according to claim 9,a nonreflection coatingis coated on the beam incident area.
 12. The optical head according toclaim 9,a dome-like projection is formed on the beam incident area. 13.The optical head according to claim 9,a prism-like projection is formedon the beam incident area.
 14. The optical head according to claim 9,agrating-like unevenness is formed on the beam incident area.
 15. Theoptical head according to claim 9,said beam incident area is incliningto said illuminating beam.
 16. The optical head according to claim 9,whereina roughened surface is formed on the beam incident area.